Turbine engines are used as the primary power source for various aircraft applications. Most turbine engines generally follow the same basic power generation process. Compressed air is mixed with fuel and burned, and the expanding hot combustion gases are directed against stationary turbine vanes in the engine. The vanes turn the high velocity gas flow partially sideways to impinge on the turbine blades mounted on a rotatable turbine disk. The force of the impinging gas causes the turbine disk to spin at high speed. Jet propulsion engines use the power created by the rotating turbine disk to draw more ambient air into the engine and the high velocity combustion gas is passed out of the gas turbine aft end to create forward thrust. Other engines use this power to turn one or more propellers, electrical generators, or other devices.
Since turbine engines provide power for many primary and secondary functions, it is important to optimize both the engine service life and the operating efficiency. Although hotter combustion gases typically produce more efficient engine operation, the high temperatures create an environment that promotes oxidation and corrosion. For this reason, diverse coatings and coating methods have been developed to increase the operating temperature limits and service lives of the high pressure turbine components, including the turbine blade and vane airfoils.
One category of conventional coatings includes ceramic materials as thermal barrier coatings, which are applied onto surfaces of turbine blades, vanes, and other components. The coated components frequently employ a metallic bond coating to improve component bonding to the ceramic thermal barrier coating. One example of a metallic bond coating is platinum aluminide. Another example of a metallic bond coating is a MCrAlY alloy wherein M is usually a metal such as Ni, Co, or Fe, such as NiCoCrAlY. Such coatings provide a bonding surface for the ceramic thermal barrier coating as a result of selective oxidation of aluminum to form an alumina (Al2O3) scale that grows very slowly at high temperatures by a diffusion process.
Current platinum aluminide and MCrAlY coatings tend to have unacceptable stability when formed on advanced third and fourth generation single crystal superalloys such as CMSX10O, EPM102, and TMS162. It is a challenge to craft bond coatings that have suitable compatibility with the superalloy substrate on which they are formed. Furthermore, interdiffusion between the bond coating and the underlying substrate material initiates the growth of a low-strength secondary reaction zone (SRZ), which penetrates the substrate and may be several times thicker than the bond coating. Also, loss of adhesion of the aluminum oxide or other oxide scale to the bond coating or to the overlying stabilized zirconia or other ceramic material may eventually limit the thermal barrier coating spalling life.
In view of the challenges associated with providing durable and effective thermal barrier coatings, there is a need for bond coatings that have improved stability on single crystal alloy substrates. There is also a need for methods of forming such coatings in a manner that prolongs adhesion of thermal growth oxide scales to both the underlying bond coating material and the overlying thermal barrier coating material.